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Area Forecast Discussion
Issued by NWS Grand Junction, CO
Issued by NWS Grand Junction, CO
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499 FXUS65 KGJT 200938 AFDGJT Area Forecast Discussion National Weather Service Grand Junction CO 338 AM MDT Sat Jul 20 2024 .KEY MESSAGES... - Temperatures are expected to remain a few degrees above normal with daily thunderstorm chances slowly decreasing through early next week. - Potential for heavy rain and localized flash flooding is possible with storms across southeast Utah and southwest Colorado today, with less coverage across the north where gusty outflow winds are more of the concern than rain. Storms will exhibit more of a gusty outflow wind threat than heavy rain as we head into the coming week. - The combination of high based storms, gusty outflow winds and lightning will also keep a threat of new wildfire starts in the picture. && .SHORT TERM /THROUGH SUNDAY/... Issued at 338 AM MDT Sat Jul 20 2024 Isolated shower/storm activity is ongoing particularly across southeast Utah and southwest Colorado this early morning with even some stray showers across the Grand Valley of west-central Colorado. This seems to be driven by the subtle remnants of a shortwave and a few vort maxes moving through the northwest flow that the HRRR seems to be picking up on fairly well. Our 00Z GJT sounding last evening indicated 0.88 inches of precipitable water with current SPC mesoanalysis indicating PWAT between 0.8 and 0.9 across the Four Corners and west- central to southwest Colorado along the CO-UT border. Models indicate that drier air will begin to advect in from the north across mainly areas north of I-70 today. It is evident that enough remnant moisture and instability exists for shower and storm activity to be not only ongoing through the night but also redevelop throughout the afternoon. The projected change in air mass is reflected well when looking at mixing ratios as values of 4 to 6 g/kg remain across southeast Utah and southwest Colorado with values reduced to 2 to 4 g/kg across northwest Colorado this afternoon. CAPE is better across the south in the realm of 200 to 400 J/kg where the better moisture exists with very little instability north of I-70. The HRRR reflects this well with the potential storm coverage showing the best potential south of I-70 this afternoon with more isolated activity north of I-70. Storms are expected to redevelop over the high terrain this afternoon with best potential across the south for the lower elevations to be impacted. Heavy rain and localized flash flooding still remains a concern across southeast Utah and southwest Colorado where the better moisture exists and especially if any storm training happens. Despite less storm coverage across the north, the HRRR does indicate some gusty outflow winds from a remnant potential storm, most likely associated with a shortwave dropping across northwest Colorado out of Wyoming early this evening, so that could be something to watch for as gusty outflow wind potential of 40 to 50 mph is possible with storms. Drier air will continue to push southward on Sunday, resulting in much less in the way of storm coverage. Locally heavy rain is still possible over the southern areas as this moisture is pushed south and the drier air slowly infiltrates from the north, so we may see more of a transition to gusty outflow winds being a primary concern rather than heavy rain. Temperatures will remain roughly the same as they have been around 5 degrees above normal today with slight cooling to near normal by Sunday as this drier air pushes in from the north. Skies could also be a bit hazy this weekend as this northerly flow brings some elevated wildfire smoke around the high pressure to our west from the fires over the Pacific Northwest and Intermountain West, especially as the drier airmass advects in from the north. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Issued at 338 AM MDT Sat Jul 20 2024 Sunday evening through Wednesday, models are in good agreement with high pressure centered in the Great Basin with ridging extending north into Alberta, and troughing over the Plains putting eastern Utah and Western Colorado under a continuing northerly flow. A deep shortwave rotates around the Gulf of Alaska low into the Pacific Northwest Monday evening, knocking the top off the ridge Tuesday, but leaving the high undisturbed in the Great Basin. A second shortwave rotates through late Wednesday after which the models diverge. Solutions range from the Gulf of Alaska low moving quickly inland across Canada with troughing along the West Coast effectively eroding the high putting the region under a southwesterly flow by Thursday, to the Gulf of Alaska low wobbling inland over British Columbia and the troughing not hitting the West Coast until late in the weekend. There is low confidence in any of these solutions beyond mid week, so we`ll just have to wait and see how this plays out. Until then, a lot of monsoonal moisture has wrapped up into the high in the Great Basin and the circulation brings periods of drier and moist air over the region for enhanced or diminished convective activity, but the current pattern of showers and thunderstorms building up over the mountains each afternoon lasting into the evening will continue through the coming week. Guidance indicates Monday will see drier air push in from the north shutting down most convection in the Uinta Basin area and northwestern Colorado and isolated showers and thunderstorm in the northern and central Colorado mountains, becoming more scattered farther south in the San Juans. A bit more moist air rotates into the region Tuesday and Wednesday producing widely scattered showers and thunderstorms across all the higher terrain of eastern Utah and Western Colorado. Thursday looks to dry out again with isolated to scattered convection over the southern mountains. With the influence of drier air early to mid week, storms should exhibit more of a gusty outflow wind threat than wetting rain. Temperatures will be near normal across the region Monday, but a warming trend will heat things about five degrees by Thursday as the high pressure to the west expands to the east into the region. The forecast guidance is hinting in a significant shift in the weather pattern with the trough moving in over the Intermountain West Friday and Saturday initiating widespread showers and thunderstorms across the region. Again, there is low confidence in this forecast at this time. Stay tuned to the forecast into early next week when we should have a better view of next weekend. && .AVIATION /06Z TAFS THROUGH 06Z SUNDAY/... Issued at 1116 PM MDT Fri Jul 19 2024 A few showers linger across the region, mostly over higher terrain. Most will end in the next few hours, but a few may hang on into the morning. Otherwise, VFR conditions with light terrain driven winds will prevail. Another round of isolated to scattered showers and thunderstorms starts around 18Z and will continue into the evening hours with most of the activity over the southern mountains and along and east of the Divide in the central Colorado mountains. && .GJT WATCHES/WARNINGS/ADVISORIES... CO...None. UT...None. && $$ SHORT TERM...MDA LONG TERM...DB AVIATION...DB